Today, more than half the world's population live in cities, and the UN expect this figure to rise to two-thirds by 2050. Already, cities are responsible for an estimated 70% of global energy use. Cities must cut their energy use in order to reduce greenhouse gas emissions, the Urban Heat Island effect and urban air pollution, and global fossil fuel (especially oil) depletion. Available approaches include shifting to alternative energy, energy efficiency improvements, and energy conservation. We find that although both increased use of renewable energy and energy efficiency improvements are desirable, they will be unable to reduce significantly fossil fuel use by 2050. This is important because many researchers argue that large reductions in oil, and to a lesser extent other fossil fuels, will be needed within the next two decades, because of their mounting energy, monetary and environmental costs. Achieving the necessary large cuts in both fossil energy use and their accompanying emissions will, we argue, necessitate energy conservation, largely though lifestyle changes.